A donor bike like that is going to cost around £1000 or even more with the upgraded brakes. Add £100 for the motor and controller, and that leaves £200 for the battery, which would have been around £500 new without the case. If it's half knackered, that's still a fair price.
If I were looking for a bike like that, I think I would be tempted. It would save a lot of messing about doing the conversion.
You can buy a new 1000w bike without rear suspension from Hammer ebikes for £1349, though you need the £119 brake upgrade.
My advice would be not to get one of these bikes unless you're sure that you can ride it at 25+ mph for a long time without drawing any attention to yourself. Even if you pass that requirement, you'll soon find that it'll bring many other problems that makes riding it it not very pleasant. 20 to 22 mph is a nice maximum speed for a bicycle. If you want 30 mph or more, get a motorbike.